NASA's Space Place: Why did it take so long to discover Uranus?
A carefully trained eye in perfect conditions can spot the dim light of Uranus without a telescope. Despite this fact, it wasn’t officially discovered until 1781—thousands of years after the other visible planets were documented. Part of the problem was not finding it, but correctly identifying it. Read all about it in the Space Place’s latest column: http://spaceplace.nasa.gov/uranus/en/.
The Space Place is a NASA educational website about space and Earth sciences and technologies. It targets upper-elementary-aged-children.